1. Field of the Invention
The present invention relates to an internal shape of a rotor for a two-bore rotary carburetor used in a stratified scavenging engine applicable to a two-stroke internal combustion engine for a power saw, lawn mower, or the like.
2. Description of the Related Art
There are conventionally known carburetors for a two-cycle engine in which the terminal end of an air channel is connected to a portion adjacent to the scavenging port of a scavenging channel for connecting the scavenging port and the crankcase of the engine. The air channel is provided with a check valve for allowing air to flow to the scavenging channel. The starting end of the air channel is connected to the inlet portion of an air intake conduit. The air channel is provided with an air control valve for varying the amount of air; and the air control valve for varying the amount of air in the air channel is provided as an integrated structure together with a fuel control valve for varying the amount of fuel in an air intake channel to the air intake channel and the air channel disposed in parallel to the carburetor main body so that the valves can rotate about an axis that traverses the two channels (see JP-A 10-252565).
In the prior-art carburetor, the same single rotor has two bores shaped as circular orifices, and the bore on the side of the fuel supply opens first during idling. To achieve this, a two-step rotor structure is adopted so that the outside diameter of the rotor is greater in the bore on the side of the air supply. The carburetor of an air-leading stratified scavenging two-cycle engine has a simple structure in which a partition is placed into the same bore, and has the merit of being able to be made more compact and allowing the engine to be designed to smaller dimensions because the height of the carburetor can be minimized. However, it is difficult to maintain a lean air-fuel ratio at idling by merely installing a partition in the same bore because the structural features of the rotor carburetor designed to aid in controlling the air-fuel ratio cause the rotor to be displaced downward at the throttle opening maintained during idling, and result in an arrangement in which the bore on the side of the air supply opens first, and the bore on the side of the fuel supply opens second.
The bore on the side of the fuel supply can be made to open first during idling if the shape of the bore on the side of the fuel supply is made symmetrically larger, but because the air-fuel ratio varies greatly when the bore on the side of the air supply starts to open in a partial state, it is difficult to control the fuel so that an optimal air-fuel ratio is obtained.